This invention relates to space transformers of buckling beam probe cards. More particular, the present invention relates to a method and apparatus for conductively contacting a buckling beam probe with a recessed terminal in a multi layer organic space transformer (MLO space transformer).
A vital part of probe apparatus for testing circuit chips are space transformers that conductively connect the back ends of the buckling beam probe beams with peripheral terminals. Probe apparatus that are used for the testing of chips with ball grid arrays have conventionally a number of parallel probes in tight assembly allowing them to contact the solder balls. In such probe apparatus, the space transformer has to provide the same array of probe end contacts as given by the tested chip""s ball grid array. In such a case, the space transformer is conveniently fabricated from the chip carrier, which is ordinarily used to permanently attach the chip after being tested. Chip carriers are part of the chip packaging as is well known to those skilled in the art.
Chip carriers conventionally feature a mask layers or solder masks with holes or material separations that expose only those conductive trace areas, which are dedicated for contacting the solder balls. The mask layer thickness and the hole diameter correspond thereby to the dimension of the individual balls in order to provide sufficient access for the soldering process. Buckling beam probes on the other hand are ideally configured with a profile thickness that is limited by the distance between the probes such that they provide a maximum strength and stiffness. Consequently, the probes have a profile thickness that may exceed the diameter of the material separation in the mask layer.
Probe beams may be slideable assembled within the probe apparatus allowing them to transmit a received testing force onto the contact surfaces of the space transformer. This results in a lateral positioning range within which the probe beam must be able to access the contacting surface of the space transformer. The probe thickness together with the eventual lateral positioning range may be close or even exceed the diameter of the material separation. Consequently, the assembly and/or operation of the probe apparatus may become significantly hampered by the task of bringing and/or securing the probe beams in contact with the contacting surfaces of the space transformer. Therefore, there exists a need for providing a contacting between the probe beams and dedicated contacted surfaces of a space transformer adapted from a chip carrier having a mask layer and material separations. The present invention addresses this need.
During the operational use of the probe apparatus, the beam probes are pressed against the solder balls on the chip under test with a certain force at which surface oxidations on the solder balls are removed. The removing of the surface oxidation is known as scrubing, which also induces a plastic deformation of the solder balls. Slideable held probe beams transmit the received contacting force via their probe ends onto the contacting surfaces of the space transformer. Chip carriers are configured to withstand mainly the contacting force induced only once and at relatively low levels during the soldering of the solder balls. As a result, the contacting force that is repeatedly induced by the probe ends during the operation of the space transformer may result in a loosening of the conductive traces that provide the contacting surface with the probe ends. The conductive traces may also be otherwise deformed, since the each contacting of the probe beam hammers the probe end onto the conductive trace resulting in an eventual gradual deformation by micro forging the contacting surface. Therefore, there exists a need for an interface structure that is configured to receive the contacting force and the hammering action of the probe end on an ongoing base without altering or destruction of the space transformers configuration. The present invention addresses also this need.
Mass produced multi-layer organic chip packages designed to permanently package chips are used as MLO space transformers of a probe apparatus having buckling beam probes. The chip packages also called chip carriers are used as preferably provided by the chip manufacturer with a solder mask layer applied on the chip carrier for the soldering of the ball grid arrays of the chips. The mask layer used for the soldering has material separations in a typical form of cylindrical holes in order to expose only dedicated areas of conductive traces beneath the soldering mask for the soldering. In the probe apparatus, probe ends are contacting the exposed trace areas. To provide a more reliable connection of the probe ends with the exposed trace areas, an interface structure is provided that is conductively and permanently connected with the exposed trace area on one end. The interface structure protrudes through the material separations sufficiently to allow a direct contacting with the probe end. The interface structure has a contacting surface in the configuration of either a bump or a planar contacting surface. The planarity of the planar contacting surface may be provided by a sanding process. As a result, the buckling beams may be assembled within a lateral positioning range without impairing a likelihood of missing the material separations, which would interrupt the conductive path towards and/or away from the buckling beams.
The interface structures are deposited in an electrochemical deposition process also known as electro plating or galvanic metal growing. The exposed trace areas are utilized thereby as seeding areas from which the interface structures grow during their fabrication. The growth process conventionally occurs in concentric direction away from the seeding area, which results in a bump initially raising above the surface level of the mask layer. This bump may be utilized as contacting surface for contacting the probe end. The bump may also be planed, for example, with a sanding process. A thereby created planar contacting surface provides a larger contacting area between the interface structure and the beam end.
In the case, where the electrochemical deposition is continued beyond the forming of bumps, the interface structure continues to grow concentrically and unidirectional as soon as the containing walls of the mask layer are overgrown. This results in a mushroom like shape of the interface structure with a flange contacting the outside mask surface surrounding the material separation. The bulky mushroom shape may be sanded down and formed in a flange having an extended planar surface area with a diameter that is bigger than that of the material separation. As a result, buckling beam probes with a section area larger than that of the material separation may be utilized for the probe apparatus.
The lateral positioning range may be compensated by the area extension of the extended planar surface such that the whole end of the buckling beam may contact regardless the position of the buckling beam within the positioning range. The contacting flange provides support during the sanding process, where shear forces threaten to deform and/or break the interface structure off the exposed trace area. The contacting flange also provides mechanical support during the operation of the probe apparatus where the buckling beam probes are mechanically moved towards and pressed onto the contacting surfaces during each test cycle.